A circuit switched (CS) domain mainly processes voice services. The signaling plane control entity in the CS domain is a mobile switching center (MSC) or a visitor location register (VLR). A base station subsystem (BSS), which is a radio access network element in a global system for mobile communications (GSM), or general packet radio service (GPRS))/enhanced data rate for GSM evolution (EDGE) radio access network (GERAN) and includes a base station controller (BSC) and a base transceiver station (BTS), is connected to an MSC through an A-interface. A radio network controller (RNC), which is a radio access network element in a universal mobile telecommunications system (UMTS) terrestrial radio access network (UTRAN), is connected to an MSC through an Iu-CS interface.
To tackle the challenges of wireless broadband technologies, an evolved packet system (EPS) is proposed. The core network of the EPS mainly includes three logical function entities, namely, a mobility management entity (MME), a serving gateway (S-GW), and a packet data network (PDN) gateway (P-GW). The MME is responsible for non access stratum (NAS) signaling, NAS signaling encryption, roaming, tracking, allocation of temporary user identities (IDs), and security. The MME corresponds to the control plane part of a serving GPRS support node (SGSN) in a current UMTS system. The S-GW is responsible for local mobility anchors and mobility anchors in a 3rd Generation Partnership Project (3GPP) system and lawful interception information. The P-GW executes functions such as policy execution, charging, and lawful interception.
Because many services of operators are provided in the CS domain, to implement the existing CS services over a long term evolution (LTE) network, a method for connecting to the CS core network through an EPS network is proposed. The P-GW in the EPS network is connected to an MSC in a traditional CS core network through an interworking function (IWF), where the IWF is capable of simulating the Iu-CS interface or A-interface, for example, through the configuration of the protocol stack corresponding to the Iu-CS interface or A-interface. When a UE intends to communicate with the MSC, the UE sends a message to the IWF and the IWF may convert the received message into the signaling format of the Iu-CS interface or A-interface and send the converted message to the MSC. Thus, the communication between the UE and the MSC is completed. Similarly, the IWF may forward a message received from the MSC to the UE. In this communication mode, the LTE network serves as an interactive channel between the UE and the IWF.
When the MSC implements functions such as tracking, charging, and lawful interception, the MSC needs to know the location information of the UE. In a circuit switched over packet switched (CSoPS) architecture, the location information of the UE is provided for the MSC by the IWF. However, the UE location information recorded in the IWF is provided by the UE. Therefore, if the IWF receives the location information reported by the UE and sends it to an associated MSC, certain risks exist. In the prior communication model, because the UE might be attacked by hackers or use illegal software, the UE is always untrusted.